Satellite images provide ‘eagle-eye’ view of grapevine canopies

NASA researcher Ian Harlen sets up a datalogger in a garlic field in the western San Joaquin Valley. The datalogger is cabled to four underground soil moisture probes and records soil moisture from these probes every hour. The data will be used to check the accuracy of satellite-based estimates of crop water use.

Research aimed at combining up-to-date weather and satellite data within a water balance model to estimate irrigation that is more precise is demonstrating some promising results.

Lars Pierce, a researcher at Cal State University, Monterey Bay, and his colleagues at the National Aeronautics and Space Administration, Ames Research Center, have been working on using satellite imaging with irrigation. The project is funded through the California Department of Water Resources and NASA.

Pierce has been working on a project in the Napa Valley since 2006 using the satellite imagery from Landsat. Using the satellite imagery, Pierce gets the normalized difference vegetation index (NDVI) by comparing red reflectance to near-infrared reflectance in the plants.

"When you take a ratio of the red to infrared, then that directly correlates with crop densities or plant densities," Pierce said. "What we do with that information then, we can get at the density of the crop."

Pierce said water use is directly related to crop density.

"The leaf surface area is responsible for transpiring the water, so the greater the leaf surface you have, the more water your crop is going to use," Pierce said.

He noted that the researchers can deduce directly from the satellite imagery an estimate of the crop coefficient. The crop coefficient is a dimensionless number (usually between 0.1 and 1.2) that is multiplied by the evapotranspiration value (ETo) to arrive at a crop evapotranspiration estimate.

"A lot of irrigation managers use this crop coefficient model, and it's pretty simple. They just take the ETo, which is the evaporation from the weather station," Pierce said.

The California Department of Water Resources has a network of weather stations throughout California called the California Irrigation Management Information System.

"CIMIS provides that ETo at each station on a daily basis, so if you know your crop coefficient, and you know your ETo, then you can multiply the two and get an estimate of crop water use for that day, or for that week, or for the month, whatever you're interested in," Pierce said. "The crop coefficient in reality changes from year-to-year just because of the differences in year-to-year weather. And so the tables are good for an average. But if you're in a warm, dry year, your crop coefficient might be ahead of where it says it is in the table or visa versa."

The satellite imagery is free to anyone, Pierce said.

"It used to cost $500 a scene, but now it's free, and it's available at the USGS (United States Geological Service) Glovis website (http://glovis.usgs.gov/), and you can go in there and search," Pierce said, adding Landsat images the entire surface of the earth every two weeks.

"We're working on a couple of different fronts with regards to actually packaging this stuff up and getting it to the grower," Pierce said.

A website is being set up that will be similar to Google Maps. A grower can zoom in, click on a particular block and find the crop coefficient.

"You could scroll through many different days over the course of the season and see, just by clicking on your particular block, how your crop coefficient changed," Pierce said.

"If the growers wanted to get more information, then what they would do is submit a map of their farm or the technical term is shapefile, but it's just basically a map that outlines the particular blocks of their farm," Pierce said. "It would go in to all the different dates where we had NDVI imagery for that farm, and it would pull a map for that individual farm and create a series of maps of that farm over a growing season.

"This provides them (growers) with an additional piece of information that can be used in making irrigation decisions," Pierce said.

Areas with limited water may be able to use their water more efficiently and actually grow more acreage with the available water, Pierce said, "and in some crops, like winegrapes, moderate water stress can improve crop quality."

The project is currently expanding into the Salinas and San Joaquin valleys, Pierce said.

"They're already pretty efficient in the San Joaquin Valley because there's good incentive to be, so this is just to try to help them dial it in a little bit more," Pierce said. "We're actually delivering kind of three different products."

The first product is going in to look at the image where the farm is located, get the NDVI and the crop coefficient on any particular day, Pierce said.

The second would be to submit a shapefile, or a boundary of the farm, or a map of the farm showing all block boundaries, in order to determine crop coefficient specifically for the fields on that farm.

At the the third level, Pierce is currently working with growers in the Napa Valley, where they actually provide him with their irrigation data.

"Any time they irrigate a block, they say, 'OK, for this block, it got so many gallons per vine.' We take that information and put that into the modeling, and we actually try to track soil moisture. Then we send them back a report, basically making an estimate of what soil moisture is by block," Pierce said, adding growers can look at the report and see where they need to irrigate or don't need to irrigate.

"Every time they e-mail a new irrigation file to the server, the server automatically retrieves the satellite imagery, and combines it with the CIMIS weather data. It actually makes a daily calculation of crop water use," Pierce said, adding it reports that automatically, generates a soil moisture report, and sends it back to the grower.

"We're setting it up right now," Pierce said, adding it will be automated, but the cost for the service has not been determined. It is possible that the service could be free, depending on long-term funding, he said.